Method of fabricating a thin film transistor

ABSTRACT

A method of fabricating a TFT includes a step of forming an impurity region for a source and a drain by simultaneously implanting and activating impurity ions. More particularly, the present invention includes the steps of forming a gate insulating layer and a gate on a predetermined and selected portion of an active layer, forming an excited region in the exposed portion of the active layer by implanting hydrogen ions to the active layer by using the gate as a mask, and forming an impurity region by implanting impurity ions heavily to the excited region which remains in an excited state.

This application is a continuation of application Ser. No. 09/286,564 filed on Apr. 5, 1999 now U.S. Pat. No. 6,281,055.

This application claims the benefit of Korean Patent Application No. 1998-12205, filed on Apr. 7, 1998, which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of fabricating a thin film transistor wherein the source and drain regions are formed by doping an active layer with impurities using a gate electrode as a mask and by activating the impurities.

2. Discussion of the Related Art

FIGS. 1A–1C show cross-sectional views of a TFT structure which illustrates a method for fabricating a thin film transistor (hereinafter abbreviated TFT) according to a related art.

In FIG. 1A, an insulating or buffer layer 13 is formed by depositing an insulating substance, such as silicon dioxide, on a transparent substrate 11, such as a glass substrate or the like, by a Chemical Vapor Deposition (hereinafter abbreviated CVD) method. An active layer 15 is formed by depositing polycrystalline silicon on the buffer layer 13 by a CVD method and by patterning the polycrystalline silicon by a photolithography process.

According to FIG. 1B, another layer of silicon dioxide is now grown or deposited across the surface of the silicon wafer. After the silicon dioxide layer covers the active layer 15, a conductive material such as aluminum or the like is deposited on the silicon dioxide layer by a CVD process.

A gate insulating layer 17 and a gate 19 are formed by patterning the conductive material and the silicon dioxide layer by a photolithography process so that they remain over a selected portion of the active layer 15, as shown in FIG. 1B.

Subsequently, ion-implanted region 21, constituting source and drain regions, is formed by heavily doping an exposed surface of the active layer 15 with impurities such as Phosphorous (P), Arsenic (AS), or the like, by using the gate 19 as a mask.

Referring to FIG. 1C, the impurities implanted in the ion-implanted region 21 are activated by application of a laser beam onto the region 21. Consequently, an impurity region 23 constituting source and drain regions is formed as the impurity ions in the region 21 are activated by the laser beam.

However, the above-described method of fabricating a TFT according to the related art involves complicated steps, such as irradiation of the impurity region with a laser beam to form source and drain regions and activation of the implanted impurity ions.

The use of laser beams for the activation of the impurity ions is expensive and difficult, and can significantly increase the risk of structural damage to the semiconductor layers because of its heat treatment intensity.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a method of fabricating a thin film transistor that substantially obviates one or more of the problems due to the limitations and disadvantages of the related art.

The object of the present invention is to provide a method of fabricating a TFT including the step of simultaneously forming an impurity region for source and drain regions and the step of implanting and activating the impurity ions in such an impurity region.

Additional features and advantages of the invention will be set forth in the description which follows and in part become apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the method of fabricating a thin film transistor comprises the steps of forming a gate insulating layer on the active layer; forming a gate on the gate insulating layer; forming an excited region in an exposed portion of the active layer by implanting hydrogen ions to the active layer by using the gate as a mask; and forming an impurity region by heavily implanting impurity ions to said excited region while the excited region remains in an excited state.

It is understood that the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide further understanding of the present invention and constitute part of this application, illustrate embodiments of the invention and in conjunction with the description serve to explain the principles of the invention.

FIGS. 1A–1C illustrate cross-sectional views of a TFT electrode structure, illustrating the method for fabricating a TFT according to a related art; and

FIGS. 2A–2C illustrate cross-sectional views of a TFT electrode structure, illustrating the method for fabricating a TFT according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiment of the present invention, an example of which is illustrated in the accompanying drawings.

FIGS. 2A–2C show cross-sectional views of a staggered-electrode or top-gate structure illustrating a method for fabricating a TFT according to the present invention.

Referring to FIG. 2A, a buffer layer 33 is formed by depositing silicon dioxide or silicon nitride on a transparent substrate 31, such as a glass substrate or the like, by a CVD method. An active layer 35 is formed by depositing undoped polycrystalline silicon on the buffer layer 33 to a thickness between about 400 and 800 Å by a CVD method or other suitable methods known to one of ordinary skill in the art. The active layer 35 is patterned by a photolithography process so that it remains on a predetermined portion of the buffer layer 33.

Alternatively, rather than forming the active layer 35 by depositing undoped polycrystalline silicon, the active layer 35 may be formed by depositing undoped amorphous silicon by a CVD method and thereafter crystallizing the amorphous silicon with laser annealing or other suitable methods known to one of ordinary skill in the art.

As illustrated in FIG. 2B, another layer of an insulating layer, namely, silicon dioxide, is deposited on the buffer layer 33 to cover the active layer 35, and a conductive material, such as aluminum or the like, is deposited on the silicon dioxide by a CVD method. A gate insulating layer 37 and a gate electrode 39 are formed by patterning or etching the conductive material and the silicon dioxide by a photolithography process so that the gate insulating layer 37 and gate electrode 39 are formed over a certain portion of the active layer 35. Preferably, the gate insulating layer 37 and the gate 39 are formed to a thickness of about 500–1500 Å and about 1500–2500 Å, respectively.

In a preferred embodiment, after the unwanted portions of the insulating layer 37 are removed, the active layer 35 is doped by ion implantation, wherein hydrogen ions are directed at the wafer, with implantation energy between about 50 and 150 KeV and with a dose of about 5×10¹⁴–5×10¹⁶ ions/cm², to alter the type and conductivity of the silicon in the desired regions.

An optimal temperature for impurity doping according to the preferred embodiment is in the range of about 200–300 degrees Celsius. In the preferred embodiment, as the size of the active region 35 increases, the time necessary for the hydrogen ion implantation correspondingly increases in order to achieve the optimal temperature between about 200 to 300 degrees Celsius. Therefore, the size of the active region or layer is proportionately related to the hydrogen implantation time required to achieve the optimal temperature.

Similarly, if the implantation energy of the hydrogen ion implantation increases, the active layer 35 heats up more rapidly.

As the implanted ions collide with the exposed surface of the active layer 35, the kinetic energy of the hydrogen ions yields thermal energy. Subsequently, the exposed surface of the active layer 35 collided with the implanted hydrogen ions is heated to the optimal temperature range falling between about 200˜300 degrees Celsius, thereby forming an excited region 41, as illustrated in FIG. 2B. As the size of the hydrogen particle is significantly small, the excited region is not damaged by the collision with implanted hydrogen particles.

According to FIG. 2C, a heavily-doped impurity region 43, which also constitutes source and drain regions, is formed by heavily doping the excited region 41 which remains in an excited state with n-typed impurities, such as P, AS, and the like. Here, the impurities are implanted in the region 41 with a heavy dose preferably between about 1×10¹⁵ and 1×10¹⁶ ions/cm².

The n-type impurity ions, once implanted into the active layer, become self-activated as the mobility of each ion particle in the excited region 41 increases. In other words, as the n-type impurity ions are implanted into the excited region 41, they simultaneously become activated, thereby quickly and efficiently forming a heavily-doped impurity region.

In short, an n-typed TFT is formed by doping the impurity region with n-typed impurity ions. In comparison, a p-typed TFT is fabricated by doping the same region with p-typed impurities, such as Boron (B), BF₂, and the like.

As explained in the foregoing description, when hydrogen ions are implanted into an active layer, such an implantation process heats the exposed surface of the active layer, thereby forming an excited region. Then, impurity ions are implanted into the excited region, forming a heavily-doped impurity region, as the impurity ions become self-activated in the excited region.

Accordingly, a method for fabricating a TFT according to the present invention does not require an additional step of laser annealing, which necessitates an expensive equipment, because the impurity region for source and drain regions is formed by a simple, single, and straightforward step of implanting hydrogen ions before implanting impurity ions for the source and drain regions to activate the impurity ions simultaneously, which accomplishes both (1) impurity doping; and (2) impurity ion activation.

It will be apparent to those skilled in the art that various modifications and variations can be made in the method of fabricating a thin film transistor of the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and equivalents. 

1. A method of fabricating a thin film transistor, comprising: forming a gate insulating layer on an active layer; forming a gate electrode on the gate insulating layer; forming an excited region in an exposed portion of the active layer by implanting hydrogen ions to the active layer by using the gate as a mask, wherein said hydrogen ions are implanted with implantation energy between about 50 and about 150 KeV; and forming an impurity region by implanting impurity ions to said excited region in a heavy dosage while the excited region remains in an excited state at a temperature between about 200 and about 300 degrees Celsius, whereby the implanted impurity ions become self-activated.
 2. The method of claim 1, wherein the gate insulating layer is formed by depositing silicon dioxide or silicon nitride on a glass substrate.
 3. The method of claim 1, wherein the active layer is formed by depositing undoped polycrystalline silicon.
 4. The method of claim 3, wherein the undoped polycrystalline silicon has a thickness of between about 400 and 800 Å.
 5. The method of claim 3, wherein the active layer is formed using chemical vapor deposition process.
 6. The method of claim 1, wherein the active layer is formed by depositing amorphous silicon and crystallizing the amorphous silicon by laser annealing.
 7. The method of claim 1, wherein the exposed portion of the active layer is formed by the steps of depositing an another layer of silicon dioxide on the gate insulating layer to cover the active layer; depositing a conductive material on the another layer of silicon dioxide; and patterning the conductive material and the another layer of silicon dioxide to form an insulating layer and to form the gate over a selected portion of the active layer.
 8. The method of claim 7, wherein the gate insulating layer and the gate comprise a thickness of about 500–1500 Å and, about 1500–2500 Å, respectively.
 9. The method of claim 1, wherein said hydrogen ions are implanted with a dose of between about 5×10¹⁴–5×10¹⁶ ions/cm².
 10. The method of claim 1, wherein said hydrogen ions are implanted in the active layer and simultaneously form the impurity region.
 11. The method of claim 1, wherein the hydrogen ion implantation time is proportionately related to the size of the active layer.
 12. A thin film transistor prepared by a process comprising: forming a gate insulating layer on an active layer; forming a gate electrode on the gate insulating layer; forming an excited region in an exposed portion of the active layer by implanting hydrogen ions to the active layer by using the gate as a mask, wherein said hydrogen ions are implanted with implantation energy between about 50 and about 150 KeV; and forming an impurity region by implanting impurity ions to said excited region while the excited region remains in an excited state at a temperature between about 200 and about 300 degrees Celsius, wherein the activation of said impurity ions implanted occurs as the step of said implanting impurity ions is performed.
 13. The thin film transistor of claim 12, wherein the gate insulating layer is formed by depositing silicon dioxide or silicon nitride on a glass substrate, and the active layer is formed by depositing undoped polycrystalline silicon. 